Calcium, phosphorus, and vitamin D metabolism are severely deranged when calcium absorption from the intestine is decreased, phosphorus excretion by the kidney is impaired and vitamin D is incompletely metabolized. This study is designed to characterize the natural history and the role phosphorus intake and vitamin D supplementation may have on calcium absorption by the gastrointestinal tract in dogs with kidney disease. The dog and the human are similar in these respects. Patent analogues of vitamin D are available and may permit successful treatment of these aberrations in man. The present study will help to establish a rational basis and schedule for treatment.